Method and apparatus for obtaining an electrocardiograph

ABSTRACT

A method of obtaining an electrocardiograph (ECG) of a patient located in a turbulent electromagnetic environment. The method includes recovering an ECG signal from a patient. The signal, including added noise, is recovered near the cardiac region as the differential signal resulting from signals delivered by two electrodes forming part of a first measurement loop. The signal further consists of simultaneously recovering from the patient a second measurement signal incorporating at least the noise, as a differential signal resulting from the signals delivered by two electrodes forming part of a second measurement loop distinct from the first measurement loop. Then, the second measurement signal is added or subtracted from the first noisy ECG signal, in real time. The second measurement signal is as a function of the polarity of the noise in the second signal relative to the noise in the first noisy ECG signal. The resulting signal is processed, converted, transmitted and displayed in real time representing the ECG essentially devoid of noise.

[0001] The instant invention concerns obtaining and processing physiological signals, especially with a view toward improving the signal/noise relationship, and has as its object a method and a device for capturing an electrocardiograph signal (ECG) which is essentially devoid of noise, particularly noise caused by variations in the local magnetic field.

[0002] In capturing physiological signals, obtaining a clear signal often poses a problem, as the signal is generally weak and drowned by background noise.

[0003] This difficulty increases when the signal is acquired in an MRI environment and when the signal involved is an electrocardiograph (ECG).

[0004] Not only is the electrocardiograph the preferred indicator of a patient's health, but it also serves as a sequencing signal, particularly for an MRI imager; for example, it may function as a signal for launching an acquisition sequence (better known as “triggering”) and/or a signal for launching an acquisition window (better known as “gating”).

[0005] In practice, the signal can be used in different ways, such as visualization, shape analysis, counting, or the like, in order to determine morphology, frequency, or other characteristics.

[0006] An electrocardiograph signal is a repetitive signal with each sequence consisting of a juxtaposition of several waves (P, QRS, T, ST).

[0007] Filtering signal noise is, of course, important in terms of monitoring. In actuality, in order to follow a patient's progress in real time, a doctor must have access to intelligible data; a noisy ECG is useless.

[0008] The detection of peaks in the QRS complex is of capital importance: first, in order to determine cardiac frequency, and second, to synchronize the capturing of images from an MRI imager with the ECG (“triggering”/“gating”). This cardiac synchronization allows each section to be excited at exactly the same moment in the cardiac cycle, and thus it provides an image of the section that is free of motion-related phenomena.

[0009] Currently the electrocardiograph signal-filtering phase is generally accomplished using analog circuits or dedicated chips with fixed characteristics and limited performances, which are not successful in effectively extricating the ECG signal from the artifacts and parasites arising from the electromagnetic turbulence that prevails, in particular, during MRI examinations.

[0010] These electromagnetic disturbances originate essentially from peaks and local changes in the principal magnetic field Bo from the action of gradients beginning during the MRI experience and introducing a noise signal in the measurement loop of the ECG signal, which may be equivalent in intensity or even louder than the ECG signal.

[0011] Note that Bo is oriented along the patient's longitudinal axis in the case of a tunnel MRI apparatus, and perpendicular to the patient's coronal plane in the case of an “open” type MRI apparatus.

[0012] Considering that the orientation of the principal field Bo corresponds to the axis Z of an orthogonal spatial index (X, Y, Z), the linear gradients at X, Y and Z can be represented as follows: ${{{grad}.X} = \frac{{Bz}}{x}},{{{grad}.Y} = {{\frac{{Bz}}{y}{and}\quad {{grad}.Z}} = \frac{{Bz}}{z}}}$

[0013] In an attempt to suppress the noise signal induced, several solutions have been proposed.

[0014] For instance, it has been proposed to submit the ECG signal to a filter that is sensitive to the increasing voltage speed of the regulated signal at a value slightly higher than the typical maximum value of the dV/dt of the ECG.

[0015] However, this process does not suppress the noise induced by the application and suppression sequences of the different gradients of elevated frequency.

[0016] It has also been proposed to derive the acquired ECG signal into two secondary signals, to delay one of the latter signals for a duration corresponding to a multiple of the period of the ECG signal, to extract the noise component from it, and then perform a subtraction between the non-delayed signal and the noise component of a preceding period.

[0017] However, this method only leads to noteworthy improvement in the ECG signal if the induced noise, and thus the electromagnetic conditions, are essentially identical over several periods of the ECG signal and if the latter is relatively regular. In practice, this is not often true and a significantly distorted ECG signal may result.

[0018] The goal of the present invention is to overcome the difficulties enumerated above.

[0019] To achieve this, the object of invention is a method of obtaining the electrocardiograph (ECG) of a patient situated in a turbulent electromagnetic environment, consisting of obtaining an ECG signal from the patient including noise from the cardiac region, as well as a differential signal resulting from signals delivered by two electrodes forming part of a first measurement loop; and further consisting of simultaneously obtaining from the patient, in essentially identical turbulent conditions, a second measurement signal incorporating at least the noise, as a differential signal resulting from the signals delivered by two electrodes forming part of a second measurement loop distinct from the first; and then either adding to or subtracting from the first noisy ECG, in real time, the second measurement signal as a function of the noise polarity in the latter relative to the noise in the noisy ECG; and finally processing, converting, transmitting and/or displaying in real time the resulting signal which represents the ECG essentially devoid of noise.

[0020] The invention will be more readily understood with reference to the following description of some preferred embodiments cited as non-limitative examples, and explained in conjunction with the attached schematic drawings, wherein:

[0021]FIG. 1 is a perspective of a transverse section of a patient, at the level of the heart, showing the implantation of electrodes according to a first variation of the method of the invention, particularly in relation to the principal field Bo in a tunnel MRI apparatus;

[0022]FIG. 2 is an overhead schematic view showing the arrangement of the electrodes of FIG. 1;

[0023]FIG. 3 is an overhead schematic view showing another electrode arrangement according to a second variation of the invention;

[0024]FIG. 4 is an overhead schematic view showing another electrode arrangement according to a third variation of the invention;

[0025]FIG. 5 is a block schema showing the different components of an ECG apparatus according to one particular realization of the invention;

[0026]FIGS. 6A and 6B are temporal diagrams of electrocardiogram signals captured during the application of an MRI sequence with and without applying the method of the invention, respectively;

[0027]FIGS. 7A and 7B are also temporal diagrams of electrocardiogram signals captured before and during the application of an MRI sequence with and without applying the method of the invention, respectively.

[0028] Thus, the method of acquiring the electrocardiograph (ECG) of a patient 1 situated in a turbulent electromagnetic environment consists, first, of recovering from patient 1 an ECG signal with additional noise S1 in the cardiac region as a differential signal resulting from the signals delivered by two electrodes 3, 3′ forming part of a first measurement loop 2.

[0029] In accordance with the invention, said method further consists of recovering from patient 1, under essentially identical turbulent conditions, a second measurement signal S2 incorporating at least the noise, as a differential signal resulting from the signals delivered by two electrodes 5, 5′ forming part of a second measurement loop 4 distinct from the first loop 2, then either adding to or subtracting from the first noisy ECG signal S1, in real time, said second measurement signal S2, as a function of the noise polarity in the latter signal relative to the noise in noisy ECG signal 1 and finally, processing, converting, transmitting, and/or displaying in real time the Resulting Signal SR, representing the ECG and essentially devoid of noise.

[0030] To ensure that at least one component representing the ECG is present in resulting signal SR, it is advantageous to provide that second measurement loop 4 is constituted and disposed so that if it acquires a signal component representing the ECG in addition to or mixed with the sound signal, said signal component representing the ECG has an opposite polarity and/or a smaller amplitude than the signal component representing the ECG recovered by first measurement loop 2.

[0031] Drawing inspiration from the creation and application of magnetic gradients during MRI experiments and the results of their cumulative actions in terms of creating and varying the intensity of local magnetic fields, the inventor has been able to determine which measurements to take in order to optimize reduction of the signal component relative to the noise induced by said gradients in the resulting ECG signal SR.

[0032] Thus, in the case of a patient 1 located in a nuclear magnetic resonance imager or a similar MRI apparatus, the apparent dimensions for magnetic flux in a plane PP perpendicular to the direction of the principal field Bo, of the first and second measurement loops 2 and 4 formed by connecting wires 2′ and 4′ of respective electrodes 3, 3′ and 5, 5′ and the material 2″, 4″ extending between the two electrodes 3, 3′; 5, 5″ in a single measurement loop 4; 4 are determined to be similar, or even identical.

[0033] In the context of this application, the resulting signal SR could advantageously be used for surveillance and monitoring of a patient and to engage the magnetic resonance imager (“triggering” and/or “gating”).

[0034] A comparison between FIGS. 1 and 2 of the attached drawings, as well as FIGS. 3 and 4, shows that depending upon the application and the constitution of the detection means used, the two measurement loops 2 and 4 could be physically separated and independent, or they could comprise a common electrode 3′, 5′.

[0035] In accordance with a first embodiment of the invention, the first and second measurement loops 2 and 4 are disposed essentially symmetrically in relation to the median plane PM of patient 1 perpendicular to the patient's coronal plane PC, while electrodes 3, 3′; 5, 5′ of the two measurement loops 2 and 4 are preferably, but not necessarily, arranged in alignment with similar spacing between the electrodes in each pair 3 and 3′; 5 and 5′ (see FIGS. 1 through 3 and 5 of the attached drawings).

[0036] The inventor is able to state with certainty that the sound signals recovered by measurement loops 2 and 4 configured and arranged as described above are essentially identical in current MRI experiments making use of magnetic field gradients.

[0037] According to a second embodiment of the invention shown in FIG. 4 of the attached drawings, the three electrodes 3, 3′; 5, 5′ forming the two measurement loops 2 and 4 are arranged in a triangular configuration near the cardiac region in such a way that the second measurement loop 4 recovers a signal component representing an ECG with opposite polarity and amplitude essentially equal to the signal component representing the ECG recovered by the first measurement loop 2.

[0038] Finally, so that it can be used in real time, especially to monitor the condition of a patient undergoing an MRI examination, the signal SR resulting from the addition or subtraction of the differential signals S1 and S2 delivered by the two measurement loops 2 and 4 can be processed by a pass-through filter 8, then used to modulate the frequency of a carrier and finally, transmitted to an apparatus 11 equipped with a means for displaying the filtered ECG signal SR, possibly after conversion into an optical signal.

[0039] It is also possible for the ECG signal to be ultimately submitted to numerical filtering using an infinite impulse response filter of the elliptical type, known as a Cauer filter, for example, a fourth order filter.

[0040] Yet another object of the present invention is an apparatus for acquiring an ECG signal in real time from a patient 1, particularly a patient undergoing an MRI examination, which can be connected to a display device 11, using the method described above.

[0041] This device consists principally of first, two modules 6, 6′ for obtaining physiological signals S1 and S2 furnished by two distinct measurement loops 2 and 4, each comprising a pair of electrodes 3, 3′; 5, 5′ placed on patient 1, with at least one of said two physiological signals S1, S2 comprising a component of a signal representing the ECG; and second, a module 7 which totals or differentiates signals S1 and S2 delivered by the two sampling modules 6, 6′; and finally, by modules 8, 9, 10, which process, or more specifically filter and convert signal SR furnished by said totaling or differentiating module 7.

[0042] Sampling modules 6, 6′ advantageously each consist of a differential instrumentation amplifier with inputs connected to the respective electrodes.

[0043] Processing modules 8, 9, 10 may consist, respectively, of a pass-through filter 8 and a frequency modulation unit 9 and an electro-optic conversion module 10 connected to a fiber optic, the other end of which is connected to an opto-electronic conversion module 11′ which is connected to or forms part of apparatus 11 incorporating the means for displaying the ECG signal.

[0044] Advantageously, all modules 6, 6′, 7, 8, 9, 10 are grouped together in a protected housing 12 forming a Faraday cage which is connected to electrodes 3, 3′; 5, 5′ with flexible, resistant connecting wires 2″, 4″ and which is not in direct contact with the patient's skin. Both sampling modules 6, 6′ may also comprise a common electrode 3′, 5′.

[0045] Moreover, the various elements and parts forming the apparatus of the invention can essentially consist of non-magnetic material, making it perfectly compatible with a magnetically sensitive, turbulent environment.

[0046] The apparatus implementing the acquisition method described above may consist of a sampling apparatus of the type described in French Patent No. 2 704 131 or in French Patent No. 2 729 071.

[0047] The advantages of the invention in terms of improved quality of the final ECG signal will be readily apparent from a study of FIGS. 6A, 6B, 7A and 7B of the attached drawings.

[0048]FIGS. 6A and 6B on the one hand, and 7A and 7B, on the other hand, represent electrocardiogram signals taken from the same patient, with the electrodes positioned in the same way and in the same electromagnetic environment.

[0049] Thus, the signals have been obtained with the electrodes placed at approximately 22 cm in front of the magnetic center, which constitutes the least favorable position (the point of maximum disruption due to gradient commutation), and with gradients of a magnitude of 16 mT/m.

[0050] The signals in FIGS. 6A and 6B have been obtained during application of an MRI sequence of the type known as “Fast Spin Echo,” while the signals in FIGS. 7A and 7B have been obtained before and during application of an MRI sequence of the type known as “Gradient Echo” with flux compensation.

[0051] The improvement in signal quality which results from applying the method of the invention will be clear from a comparison of FIG. 6A to FIG. 6B, and also FIG. 7A to FIG. 7B.

[0052] The invention remains valid even if the electrode arrangement is changed.

[0053] Obviously, the invention is not limited to the embodiments described and shown in the attached drawings. Various modifications are possible, particularly from the point of view of the constitution of the different elements, or the substitution of equivalent techniques, without departing from the scope of protection of the invention. 

1. A method of obtaining the electrocardiograph (ECG) of a patient located in a turbulent electromagnetic environment, consisting of recovering an ECG signal from the patient with added noise near the cardiac region as the differential signal resulting from the signals delivered by two electrodes forming part of a first measurement loop, characterized in that it further consists of simultaneously recovering from the patient (1) in essentially identical turbulent conditions, a second measurement signal (S2) incorporating at least the noise, as a differential signal resulting from the signals delivered by two electrodes (5, 5′) forming part of a second measurement loop (4) distinct from the first (2), then adding to or subtracting from the first noisy ECG signal (S1), in real time, said second measurement signal (S2) as a function of the polarity of the noise in said second signal relative to the noise in the noisy ECG signal (S1) and, finally, processing, converting, transmitting and/or displaying in real time the resulting signal (SR) representing the ECG, which is essentially devoid of noise.
 2. A method of acquisition according to claim 1 characterized in that the second measurement loop (4) is composed and arranged in such a way that, if it acquires a signal component representing the ECG that is in addition to or mixed with the noise signal, said signal component representing the ECG has an opposite polarity and/or an inferior amplitude to the signal component representing the ECG obtained by the first measurement loop (2).
 3. A method of acquisition according to either claim 1 or claim 2 characterized in that, in the case where a patient (1) is located inside a nuclear magnetic resonance imager or similar MRI analysis apparatus, the apparent dimensions for magnetic flux, in a plane (PP) perpendicular to the direction of the principal field (Bo) of the first and second measurement loops (2 and 4) formed by the connecting wires (2′ and 4′) of the respective electrodes (3, 3′; 5, 5′) and the materials (2″, 4″) extending between the two electrodes (3, 3′″ 5, 5′) of a single measurement loop (2; 4) are similar or even identical.
 4. A method of acquisition according to any one of claims 1 through 3 characterized in that the first and second measurement loops (2 and 4) comprise a common electrode (3′, 5′).
 5. A method of acquisition according to any one of claims 1 through 4 characterized in that the first and second measurement loops (2 and 4) passing through the electrodes are arranged essentially symmetrically relative to the median plane (PM) of the patient (1) perpendicular to the patient's coronal plane (PC).
 6. A method of acquisition according to claim 5 characterized in that the electrodes (3, 3′; 5, 5′), either separate or with a common electrode, of the two measurement loops (2 and 4) are arranged in alignment.
 7. A method of acquisition according to any one of claims 1 through 4 characterized in that the three electrodes (3, 3′; 5, 5′) forming the two measurement loops (2 and 4) are arranged in a triangular configuration near the cardiac region in such a way that that the second measurement loop (4) obtains a signal component representing the ECG which has the opposite polarity and is essentially equal in amplitude to that of the signal component representing the ECG obtained by the first measurement loop (2).
 8. A method of acquisition according to any one of claims 1 through 7 characterized in that the signal (SR) resulting from the addition or subtraction of the differential signals (S1 and S2) delivered by the two measurement loops (2 and 4) is processed by a pass-through filter (8), then used for frequency modulation of a carrier and finally, transmitted to an apparatus (11) equipped with a means for displaying the filtered ECG signal (SR), possibly after conversion into an optical signal.
 9. A sampling apparatus for obtaining the ECG signal of a patient in real time, particularly a patient undergoing MRI examination, which apparatus can be connected to a display device; and in particular, an apparatus using the method according to any one of claims 1 through 8, characterized in that it consists principally of first, two modules (6, 6′) for acquiring physiological signals (S1 and S2) furnished by two separate measurement loops (2 and 4), each comprising a pair or electrodes (3, 3′; 5, 5′) placed on the patient (1), at least one of said physiological signals (S1, S2) comprising a signal component representing the ECG; and second, of a module (7) which totals or differentiates the signals (S1 and S2) delivered by the two acquisition modules (6, 6′) and finally, by the modules (8, 9, 10) which process, i.e., filter and convert, the signal (SR) delivered by said totaling or differentiating module (7).
 10. A device according to claim 9 characterized in that all the modules (6, 6′, 7, 8, 9, 10) are grouped within a protected housing (12) forming a Faraday cage which is connected to the electrodes (3, 3′; 5, 5′) by resistant, flexible connection wires (2″, 4″) and which is not in direct contact with the patient's skin.
 11. A device according to either of claims 9 and 10 characterized in that the two acquisition modules (6, 6′) comprise a common electrode (3′, 5′).
 12. A device according to any one of claims 9 through 11 characterized in that the different elements and portions forming said device consist essentially of non-magnetic materials, making it completely compatible for use in a magnetically sensitive, turbulent environment. 